Shaye Cohen now goes to discuss social mechanisms that made (or might have made) Jews distinctive in the first century BCE and CE.

If the Jews of antiquity looked like evryone else, spoke like everyone else, were named like everyone else and supported themselves like everyone else, how did you know a Jew in antiquity when you saw one?

He is going to suggest two methods which give you confidence, but equally, perhaps not.

You might reasonably concluded that people you see associating with Jews are themselves Jews, and you might reasonablty conclude that people you see observing Jews laws are Jews.

"If the Jews of antiquity looked like evryone else, spoke like everyone else, were named like everyone else and supported themselves like everyone else, how did you know a Jew in antiquity when you saw one?"

I speculate that some Jews did not follow all of the Law. There were prohibitions to intermingling races/cultures as well as requirements of dress, etc...

"You might reasonably concluded that people you see associating with Jews are themselves Jews, and you might reasonablty conclude that people you see observing Jews laws are Jews."

A reasonable conclusion assuming an integrated society. However, by design, Jews were/are? a called out people. Abraham did not belong to God until he did as God requested and moved. In antiquity, people lived and worshipped together as a unified society. Faith was in survival which depended on compliance to society norms.